1. Field of the Invention
The present invention relates to a method for the manufacture of a pack, particularly intended for undergoing preservation treatment, from at least one film comprising two plastic layers, an upper and a lower, which form the outer and inner faces of the said pack, and a light-metal central layer sandwiched between the said upper and lower layers.
It also relates to a flexible pack, particularly intended for undergoing preservation treatment, especially after it has been filled and closed, comprising two flanks connected at their lateral edges, each flank comprising plastic inner and outer faces and a light-metal central layer sandwiched between the two inner and outer faces, the said pack being produced particularly by such a method.
The invention is used especially advantageously in the production of pouches or bags containing food substances, particularly food substances for animals, such pouches or bags being intended to be closed hermetically by welding and treated thermally after being filled and closed.
2. Description of the Prior Art
At the present time, such packs have various forms.
They may be flat with three or four welds or gusseted with three welded sides or three welded sides and a gusset-shaped welded bottom.
Such packs may also comprise two lateral gussets or two gussets positioned at the top and bottom of the pack with welded lateral edges.
These packs may be sterilized by being heated in a humid atmosphere or under pressure.
In order to withstand such thermal treatment, they are produced from one or more films having a special structure.
More especially, they are usually manufactured from one or more films, each film comprising an upper layer and a lower layer which are intended for forming the outer and inner faces of the said packs, the said outer and inner faces being produced from plastic, and a central layer sandwiched between the said upper and lower layers, the said central layer being produced from light metal.
In general, the upper layer is a polyethylene terephthalate layer combined or not with a polyamide layer. The central layer is produced from aluminium and the lower layer is a polypropylene layer combined or not with a polyamide layer.
Between each layer is provided an adhesive coating making it possible to connect the various layers to one another.
Moreover, the polyethylene terephthalate layer may be printed on its inner face.
The polyethylene terephthalate layer serves generally as a printing support and gives the packet""s external appearance.
The aluminium central layer forms a barrier to gas, particularly to oxygen and steam, so as to insulate from outside the substances which are contained within the pack.
The polyamide layer forms an anti-perforation layer, when this is necessary in accordance with the subsequent uses of the pack, and the polypropylene layer serves as a welding agent for making the pack, gives the pack general mechanical stability and ensures sealing at the welds of the said pack.
Such packs may contain solids of a moist nature, liquids or else a combination of solid and liquid.
Some packs may comprise a start-off-tear notch on their lateral edges. In this case, this notch is made in a lateral weld of the pack that makes it possible to open the latter by tearing.
These already known packs have some disadvantages, particularly with regard to the method of opening them.
In fact, it is not very practical to use a cutting tool of the type of a pair of scissors or a knife, since, for example, this tool may be soiled by the product contained within the pack.
When the pack can be opened by tearing, started with the aid of a start-off-tear notch, the tearing force has to be relatively great and sometimes makes it necessary to employ a tool.
In fact, the tearing force of a multi-layer film usually depends on the tearing force of the materials forming the pack and on the adhesive force between the various layers forming the said pack, and these forces may be adversely affected after the latter has undergone preservation treatment.
The tearing direction is generally propagated in the flanks of the pack in a non-oriented manner. Thus, either partial opening of the pack is obtained, or there is a tear below the level of the product contained in the pack, thereby causing this product to flow out undesirably.
In order to attempt to overcome the above-mentioned disadvantages, mainly regarding the opening of these packs, packs have already been provided which comprise on the outer faces of their flanks a precut made by laser.
Such packs are described, in particular, in the documents EP-540,184, EP-473,517, U.S. Pat. No. 5,158,499 and U.S. Pat. No. 5,630,308.
However, the main disadvantage of this precut, carried out by applying a laser beam to the outer face of the flanks of the pack, is that it adversely affects the external appearance of the pack, once the latter has been produced.
Moreover, document U.S. Pat. No. 5,000,321 discloses a pack which comprises on its outer and inner faces weakening zones which are produced by means of a laser beam. These weakened zones are obtained by thinning the layers forming the outer and inner faces of the said pack.
However, such a system is not entirely satisfactory, since the weakening of the layers forming the outer and inner faces of the pack may be insufficient to bring about an exact and directed opening of the pack.
The present invention, then, provides a new method for the manufacture of a pack, particularly intended for undergoing preservation treatment, which makes it possible to produce a pack having easy and guided opening over its entire width.
More especially, according to the invention, there is provided a method for the manufacture of a pack, particularly intended for undergoing preservation treatment, from at least one film comprising two plastic layers, an upper and a lower, which form the outer and inner faces of the said pack, and a light-metal central layer sandwiched between the said upper and lower layers. It is characterized in that, prior to the operations to make the said pack, a laser beam is applied solely to the lower layer of the said film, the said lower layer forming the inner face of the said pack, in order to produce, virtually in the entire thickness of the said lower layer, at least one perforation forming at least one precut line.
Within the scope of the present invention, by preservation treatment will be meant any treatment known to the average person skilled in the art, especially thermal treatments, such as conventional sterilization, flash sterilization and pasteurization.
Thus, according to the invention, the lower film layer forming the inner face of the pack is embrittled along a defined line by the application of a laser beam. The laser beam is absorbed by the plastics forming the lower layer of the film and is reflected totally by the light-metal central layer. In the region of the impact of the incident beam, the materials are heated, melt and evaporate. This results in a perforation of the lower layer as far as the light-metal layer along the laser beam application line.
When the pack produced according to the manufacturing method is opened, the tear is propagated along the precut line thus made and continues along this line over the entire width of the pack.
Directed and exact opening of the pack is thus obtained.
According to one embodiment of the method according to the invention, each perforation is a continuous perforation which extends over part of the width of the film.
According to another preferred embodiment of the method according to the invention, each perforation is a succession of micro-perforations forming a precut line.
The perforation on the lower film layer forming the inner face of the pack advantageously makes it possible to avoid adversely affecting the central layer and the outer face of the pack and to maintain the long-term sterilization performances of the pack thus produced.
The production of micro-perforations in the form of dots makes it possible to avoid adversely affecting the strength of the pack thus produced.
Moreover, the production of micro-perforations in the form of dots makes it possible to protect the light-metal central layer.
In fact, a continuous perforation over the entire width of the film would subsequently make it possible to fold the film along the precut line, and this would result in the light-metal central layer breaking by exceeding the limit of elasticity of this layer, the consequence of this being a loss of efficiency of this central layer which has to form a barrier to oxygen and water vapours, and a risk of subsequent perforation of the multi-layer film, from which the pack is produced.
According to the preferred embodiment of the method according to the invention, by which at least one succession of micro-perforations having at least one precut line is produced in the entire thickness of the lower layer of the film, there may be provision for this precut line to be made over the entire width of the film.
Alternatively, each precut line may be made only over part of the width of the film, leaving free of micro-perforations the two longitudinal edges of the film which are intended to be welded in order to make the said pack.
If each precut line extends over the entire width of the film, the fact that this precut line consists of micro-perforations in the form of a line of dots makes it possible, by virtue of its interrupted nature at microscopic level, to carry out sealing welding of the edges of the film in order to produce the pack.
The variant, according to which the precut line is produced by means of micro-perforations in the form of a line of dots over only part of the width of the film, leaving the two longitudinal edges of the film free of micro-perforations, is intended more especially for when the materials forming the lower layer of the film require a density of micro-perforations such that the sealing of the welds produced on the longitudinal edges of the film can no longer be ensured.
Thus, in this case, it is preferable for the precut line not to extend in the region of the edges, so that the welds subsequently made are not affected by the precut, and the sealing of the pack is maintained.
According to a variant of the method according to the invention, two parallel perforations forming two parallel precut lines slightly spaced from one another are produced simultaneously in the entire thickness of the said lower layer of the film.
These two perforations are advantageously two successions of micro-perforations.
In this case, the spacing between the two parallel precut lines may preferably be approximately between 2 and 5 mm.
In order to produce at least two parallel precut lines simultaneously, a single laser source may be used, combined with an optical device for devising the laser beam coming from the said source into at least two identical beams.
Two laser sources may also be used, supplying two identical laser beams in order to produce the two parallel precut lines simultaneously.
If, according to the invention, a succession of micro-perforations is produced, the density of the micro-perforations is constant over the entire length of each precut line produced and is preferably between 3 and 6 micro-perforations per millimeter.
The power of the laser beam used is set as a function of the characteristics of the materials forming the said film.
Of course, in an embodiment of the method according to the invention, by which the film is wound on reels and fed continuously, the laser beam is then applied, at specific locations on the said film, solely to the lower layer of the latter, in order to produce at each specific location, virtually in the entire thickness of the said lower layer, at least one perforation forming at least one precut line.
Advantageously, this perforation is a succession of micro-perforations forming at least the precut line.
In the method according to the invention, after the step of welding the edges of each pack, a start-off-tear notch may be produced in the latter, the said notch being positioned on at least one lateral edge of the said pack between the two parallel precut lines.
Thus, the start-off tear is made visible by the notch cut in the region of a lateral weld of the pack and is conducive to tearing the materials forming the said pack.
In this case, the tear is first propagated in the weld zone in the direction of the opposite weld of the pack, without its direction being controlled. The tear is then propagated in the pack flank comprising the two parallel precut lines, until it meets one of the two precut lines.
The tear continues along this precut line which guides opening as far as the opposite side of the pack. This results in a rectilinear opening extending from one lateral weld of the pack to the other, making it possible to empty the content of the pack easily.
The invention also provides a flexible pack, particularly intended for undergoing preservation treatment, especially after it has been filled and closed, comprising two flanks connected at their lateral edges, each flank comprising plastic inner and outer faces and a light-metal central layer sandwiched between the two inner and outer faces, characterized in that at least one inner face of the said pack comprises, in its entire thickness, at least one perforation forming at least one precut line located in the upper part of the said pack.
Advantageously, each perforation is a succession of micro-perforations forming a precut line.
As regards a continuous perforation, this extends only over part of the width of the pack.
In an embodiment of the pack according to the present invention, this pack comprising welds on its lateral edges, each precut line consisting of a succession of micro-perforations extends over the entire width of the said pack.
Alternatively, it is conceivable for each precut line to extend over only part of the width of the said pack, being interrupted in the region of its lateral welds.
The pack according to the present invention may, on at least one of its inner faces, comprise two parallel precut lines at a distance of approximately 2 to 5 mm from one another. In this case, it comprises at least one start-off-opening notch located on one of its welded lateral edges between the two parallel precut lines.
Advantageously, the pack according to the present invention comprises on each inner face, in the entire thickness of the inner face, at least one perforation forming at least one precut line.
The central layer of each flank of the said pack according to the present invention is preferably produced from aluminium. The inner face of each flank of the said pack according to the invention comprises a polypropylene layer which may be combined or not with a polyamide layer. The outer face of each flank of the pack according to the invention comprises a polyethylene terephthalate layer combined or not with an additional polyamide layer.
The pack according to the invention is preferably produced by the method according to the invention.